Investigations of the interaction of light and hematoporphyrin (HP) for the killing of cancer cells has been carried out. The enhancement by selected agents to augment the photo-induced HP killing effect has been evaluated. Previous work indicated that for leukemic L1210 cells(1 million cells/ml.) in vitro the most efficient wavelength for killing (assessed by trypan blue exclusion) was 503 nm, which corresponds to one of the HP absorption peaks. In addition, the presence of succinyl acetone in the medium (in addition to the HP) was found to enhance the effectiveness of this photo-induced killing. Additional agents which are known to alter membrane properties, chloroquine and dibucaine, have also been found to produce an enhanced effect: e.g. addition of 10 M chloroquine to the medium produced an order of magnitude increase in cell death after 3 days relative to HP alone. The mode of action of this enhancement has not been determined. Other attempts, using the single oxygen trap, -carotene, and the anti-oxidants, vitamins C and E, to implicate singlet oxygen as the cytotoxic agent have proven inconclusive. In vivo studies on mice of inter-peritoneal irradiation using fiber optic probes connected to an argon ion (514nm) laser head are being established. This laser line, as noted above, is close to a major absorption peak which has been found particularly effective for cell killing. The experience gained with the enhancing agents of the photoinduced HP effect will be applied where appropriate to this study.